With over 100 million tons produced annually, O2 is one of the most widely used commodity chemicals in the world.1 Its potential utility in processes associated with the reduction of carbon dioxide emissions from fossil fuel-burning power plants, however, means that the demand for pure O2 could grow enormously. For example, when implementing pre-combustion CO2 capture, pure O2 is used for the gasification of coal, which produces the feedstock for the water-gas shift reaction used to produce CO2 and H2.2 In addition, oxyfuel combustion has received considerable attention for its potential utility as an alternative to post-combustion CO2 capture. Here, pure O2 is diluted to 0.21 bar with CO2 and fed into a power plant for fuel combustion. Since N2 is absent from the resulting flue gas, the requirement for post-combustion separation of CO2 from N2 is eliminated.3 
The separation of O2 from air is currently carried out on a large scale using an energy-intensive cryogenic distillation process.4 Zeolites are also used for O2/N2 separation,5 both industrially and in portable medical devices. However, this process is inherently inefficient as the materials used adsorb N2 over O2 with poor selectivity. By employing materials that selectively adsorb O2 and can operate near ambient temperatures, lower energy and capital costs could be realized. Metal-organic frameworks (“MOFs”), which have already received considerable attention for applications in gas storage6 and separation,7 represent a promising new class of potential O2 separation materials.
The energy cost associated with the separation of hydrocarbons, as currently carried out at enormous scale via cryogenic distillation, could potentially be lowered through development of selective solid adsorbents that operate at higher temperatures and lower pressures. As a consequence of the similar sizes and volatilities of the hydrocarbons, separations, for example, of olefin/paraffin mixtures, such as ethylene/ethane and propylene/propane, must currently be performed at low temperatures and high pressures, and are among the most energy-intensive separations carried out at large scale in the chemical industry. Because these hydrocarbon gaseous mixtures are produced by cracking long-chain hydrocarbons at elevated temperatures, a substantial energy penalty arises from cooling the gases to the low temperatures required for distillation. Thus, tremendous energy savings could be realized if materials enabling the efficient separation of hydrocarbons at higher temperatures, than currently used in distillation, and atmospheric pressure were achieved.
Current competing approaches for separating hydrocarbons include membrane designs, organic solvent-based sorbents, as well as porous solid adsorbents featuring selective chemical interactions with the carbon-carbon double bond in olefins. In this latter category, MOFs, which offer high surface areas, adjustable pore dimensions, and chemical tenability, have received considerable attention as adsorbents in gas storage and separation applications, with particular emphasis on the dense storage of methane and hydrogen, and on the efficient removal of carbon dioxide from flue gas and natural gas deposits. More recently, MOFs represent a promising new class of potential hydrocarbon separation materials.
In addition to the separation of binary olefin/paraffin mixtures, there is tremendous current interest in separating ethane, ethylene, and acetylene from methane for the purification of natural gas. Indeed, a number of porous materials are able to selectively separate methane from mixtures including C2 hydrocarbons (ethane, ethylene, and acetylene). These materials, however, are unable to simultaneously purify the ethane, ethylene, and acetylene being removed from the gas stream. A separation process that utilizes the same adsorptive material for the separation and purification of all four components of a C1/C2 mixture could potentially lead to substantial efficiency and energy savings over current processes.
Ethylene produced in a naphtha cracker contains an impurity of approximately 1% acetylene. However, there are strict limitations to the amount of acetylene that can be tolerated in the feed to an ethylene polymerization reactor. The current technology for this purpose uses absorption with liquid DMF, but the use of solid adsorbents could potentially provide an energy-efficient alternative.
In addition, early efforts in developing metal-organic framework catalysts have largely focused on new synthetic methods for incorporating catalytic functionalities onto the pore surface, as well as proof-of-concept studies, such as the heterogenization of well-known homogeneous catalysts or simple acid/base activation of substrates. While these examples demonstrate the viability of metal-organic frameworks as heterogeneous catalysts, they provide little improvement over existing systems and do not take full advantage of properties unique to metal-organic frameworks, including the ability to design specific and spatially separated active sites. In particular, framework incorporation of reactive transition metal intermediates, such as metal-ligand multiple bonds or low-coordinate metal centers, is a promising area that has yet to be explored. In principle, redox catalysis involving the formation of metal species in unusual coordination environments, geometries, and/or oxidation states that are entirely unfeasible in homogeneous systems could proceed easily in the context of metal-organic frameworks wherein each metal center is held fixed and isolated.